Southern African Natural Gas Advisory Service Service

Compressed Natural Gas (CNG)

Compressed Natural Gas (CNG) is made by compressing natural gas to less volume that it has at standard atmospheric pressure. It is stored and transported in pressurized containers at a pressure of between 200 and 245 bar (2900 - 3600 pounds per square inch: PSI). Compressors and de-compressors for this purpose are less costly and more economical when smaller volumes of natural gas are involved that LNG liquefaction and regasification plants that are expensive to construct and operate.

The volumetric energy density of CNG is approximately 25% of diesel and 42% of LNG. There is frequently confusion between CNG and LNG.

The important difference is that CNG is compressed natural gas stored as gas at high pressure, whereas LNG is uncompressed gas produced in a liquid state by a low temperature production process that is stored and transported

' frozen ' as a liquid. CNG is also sometimes confused with liquid petroleum gas (LPG) which is liquefied propane which can be compressed without freezing (unlike natural gas containing mostly methane) for storage and supply to users in pressurized cylinders and vessels.

An excellent example of how CNG is used is in transport although it has a wide range of other applications. Transport is described in this section to further explain CNG, the major features and how it may be used.    

Across the world there were more than 12 million natural gas vehicles in 2010 using compressed natural gas. Pakistan, Iran, Argentina, Brazil, India and the Asia-Pacific region all have large numbers of CNG vehicles. Europe and U.S. also have dual-fuel cars available that use CNG and petrol with the driver simply switching between fuels as necessary.

There are also many public transport vehicles powered by CNG and railroads use CNG for locomotives.  These railway trains are commonly diesel locomotives converted to use CNG generators instead of diesel generators to provide electricity to drive the train.

Vehicles using CNG require more space than with petrol or diesel to accommodate the gas storage container.  This can sometimes be a problem when retrofitting existing vehicles and may lead to a major reduction in the ' trunk ' or ' boot ' space available after accommodating the CNG cylinder/s. Apart from this the conversion of vehicles to use CNG is a straightforward, inexpensive and proven procedure.  Cars that are specifically designed to use CNG that are factory-built for this purpose readily solve the additional storage difficulty.

The benefits of CNG for vehicles are numerous with the most important being lower operating costs, less pollution and greater energy efficiency than petrol or diesel powered vehicles. CNG mixes efficiently with air with it being a gaseous fuel rather than liquid fuels and this greatly improves the efficiently of combustion in the engine.

CNG also emits greatly reduced  pollutants than petrol and diesel such as carbon dioxide, carbon monoxide, nitrogen oxides, sulphur oxides and unburned hydrocarbons. Engine wear is lower with CNG and vehicles benefit from lower maintenance costs than conventional ' oil ' powered vehicles. CNG powered vehicles are also generally regarded to be safer due to the lower flammability of CNG than petroleum products.